A focused study of fundamental & functional materials using beamline techniques to investigate magnetic and crystal structure interactions

Arnold, Robert (2021). A focused study of fundamental & functional materials using beamline techniques to investigate magnetic and crystal structure interactions. University of Birmingham. Ph.D.

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Abstract

The development of functional materials is often built upon a cornerstone of knowledge provided by study of the underlying physics in more fundamental materials - understand- ing mechanisms and causality can pave the way to optimisation of application. Large scale beamline facilities offer unique opportunities to probe length scales and structures outside the remit of traditional laboratory equipment. Here, a two part study into magnetostrictive materials is presented - a small-angle neutron scattering study of Fe $_{68.8}$ Pd $_{31.2}$ and a synchrotron X-ray diffraction study of Tb $_{1−x}$ Dy $_x$ Fe $_2$ alloys. For iron-palladium, the results confirm the transition temperature from face centred cubic to face centred tetragonal and highlight a peculiarly high degree of scattering for the latter, hypothesised to arise from enhanced magnetisation. Furthermore details of an anisotropy along the [110] directions is shown, alongside the presentation of the physical sizes of the underlying scattering feature, before finally showing evidence for a direction sensitive high field, nanoscale feature. Morphotropic phase boundaries, composition and temperature sensitive dividing regions between tetragonal and rhombohedral phases, are found to be linked to physical property enhancement for a range of functional materials. This diffraction study, performed for a range of Tb $_{1−x}$ Dy $_x$ Fe $_2$ samples with 0.56<x<0.87, produces an expanded phase diagram, which shows the morphotropic phase boundary is rapidly fluctuating with sample composition, while also providing clear information regarding the makeup of the morphotropic phase boundary. The high temperature measurements show a slow melting toward ’cubic-like’ reflection behaviour, likely caused by a reduction in lattice distortion, consequently decreasing the degree of peak splitting below that which is detectable. Lastly, evidence is presented for the discovery of a low temperature, low symmetry phase which is found to coexist with the rhombohedral phase at temperatures below 100 K.